It is good to facilitate planning meetings by an external person. This is not a must but it helps. As Sam said it is desirable to have a distribution both between individual and collective stakeholders. The ideal would be not to put pounds Relationship time between stages. So to keep the file of ideas. Needless to say, get rid of all other occupations even minimal to devote himself entirely to the work. Even when they are assigned individual tasks.
In our case, we arrived at following several specific meetings outside our usual premises. This would mean that one should not minimize the cost engendered such a SP process. This is a preliminary exercise. Be sure to have the financial resources to cover all stages of the process and also ensure the availability of all stakeholders in steps.
The SP process is both demanding but unavoidable.
Good road to those who will engage in it